Smooth underwater hull surfaces are essential for ensuring optimum performance of ships, and even a thin layer of slime, which develops rapidly, creates additional friction. Considering the high cost of fuel and the high utilisations of ships, even a marginal additional friction has a significant negative impact on the total fuel cost.
The current anti-fouling paint systems cannot prevent the formation of slime and other fouling within the usual docking intervals; thus, there is a need for underwater hull cleaning between dockings to minimise the formation of slime, fouling and other friction-enhancing objects on the underwater hull.
It is known in the prior art to clean the underwater hull. However, several disadvantages have been observed with these known techniques, namely:
a) The anti-fouling layer, i.e. paint, on the underwater hull may be sporadically damaged, or even in some circumstances completely removed, whereby the underwater hull is exposed to the maritime environment, and thereby there is a huge risk for an increasing future growth rate of the slime on the underwater hull. This is most often the case when mechanical cleaning, that is the application of brushes and similar means, is used.b) The cleaning of the underwater hull often contaminates the environment with the residues of the anti-fouling layer in the slime. The slime itself can furthermore be harmful to the environment as it may contain non-indigenous species.c) The cleaning operation is time-consuming, and may in many circumstances exceed the ships' usual turn-around time in the harbours, which may have severe consequences for the ship-owners due to the fact that they cannot then keep to their schedules.d) The cleaning operations are often performed manually by divers, and the underwater environment provides unfavourable working conditions for the divers. Since the working conditions for the divers are unfavourable, the divers often have an urge to finish the cleaning operations rapidly, which in some circumstances may result in the quality of the cleaning being non-satisfactory.
From WO 2012/074408 A2 a submergible cleaning system is known.